What to Do if Your Boiler Loses Pressure When Heating is On

As a Gas Safe registered engineer with over a decade of experience servicing boilers across Huddersfield, I encounter this frustrating problem regularly. Nothing's quite as concerning as watching your boiler pressure gauge drop from a healthy 1.5 bar to zero whilst your heating system is running.

Let’s look at why this happens and what you can do about it to save you from cold nights and expensive emergency callouts.

When your boiler loses pressure specifically during heating operation, it's telling you something important about your system's health. Let me walk you through the most common causes and practical steps you can take to diagnose and address this issue.

Understanding the Root Causes

Faulty Expansion Vessel: The Primary Culprit

In my experience, a faulty expansion vessel is the most common reason for pressure loss during heating cycles. This crucial component manages the natural expansion of water as it heats up in your system. When water temperature rises from cold to operating temperature (around 60-80°C), it expands by approximately 4%.

A properly functioning expansion vessel contains nitrogen gas separated by a flexible diaphragm. When the system water expands, this nitrogen compresses to accommodate the volume change. However, if the vessel has lost its nitrogen charge or the diaphragm has failed, it cannot manage this expansion effectively.

What happens next is a domino effect: as water expands with nowhere to go, system pressure spikes dramatically. This triggers the pressure relief valve (PRV) to discharge excess water, which then results in the pressure drop you observe.

Pressure Relief Valve Issues

The pressure relief valve acts as your system's safety mechanism, typically opening when pressure exceeds 3 bar. However, when this valve becomes faulty or contaminated with debris, it may leak intermittently when heating activates. I've found that older PRVs, particularly those over five years old, can develop sealing issues that cause pressure drops during heating cycles.

Hidden System Leaks

Even microscopic leaks in radiators or pipework become more problematic when heating operates. As pipes and joints expand with heat, small gaps can widen, allowing water to escape. These leaks are often invisible during cold periods but become apparent when the system heats up.

Pump Speed Complications

An often-overlooked cause is excessive pump speed. When circulation pumps run too fast, they can create air bubbles that eventually form hydrogen gas. This gas naturally escapes through automatic air vents, gradually reducing system pressure over heating cycles.

Diagnostic Steps You Can Take

1. Monitor the Pressure Gauge

Start by checking your pressure gauge when the system is completely cold. Normal pressure should read between 1-1.5 bar. If it shows zero, your system definitely needs repressurising. However, if it reads correctly when cold but drops during heating, you're dealing with one of the issues I've described above.

2. Conduct a Visual Inspection

Examine all visible pipework, radiator valves, and radiator joints for signs of leakage. Look for:

• Rust stains or corrosion

• Damp patches around joints

• Water stains on walls or floors

• White calcium deposits (indicating dried water leaks)

3. Check the PRV Outlet

Locate your pressure relief valve discharge pipe – typically a 22mm copper pipe exiting your property externally, often near where your boiler flue exits. Check if water is dripping from this pipe or if there's evidence of recent discharge (water stains, makeshift collection containers).

4. Assess Radiator Performance

If radiators are hot at the bottom but cold at the top whilst pressure drops, trapped air is likely your culprit. This air eventually escapes through automatic air vents, reducing overall system pressure.

5. Attempt Repressurisation

If you're comfortable doing so, use your boiler's filling loop to restore pressure to 1-1.5 bar. The filling loop is usually located beneath your boiler – a flexible braided hose connecting two isolation valves. Slowly open both valves whilst watching the pressure gauge, closing them once you reach the correct pressure.

Important: If pressure drops again within 24 hours, stop attempting to refill and call a professional.

When to Call a Professional

Based on my experience with hundreds of similar cases, professional intervention is essential if:

• Pressure continues dropping after repressurisation

• You suspect expansion vessel failure (requires pressure testing)

• The PRV needs replacement or adjustment

• You're uncomfortable performing basic checks

These components require specialist tools and expertise. Expansion vessel replacement, in particular, involves working with pressurised nitrogen gas and requires Gas Safe registration for safety compliance³.

Prevention and Long-term Solutions

Regular annual boiler servicing can identify these issues before they become problematic. During services, I always check expansion vessel pre-charge pressure and PRV operation. The Energy Saving Trust recommends annual professional maintenance to prevent such issues and maintain system efficiency.

Modern boilers include diagnostic features that can help identify pressure-related problems early. If your boiler displays fault codes alongside pressure loss, note these down for your heating engineer – they provide valuable diagnostic information.

Boiler pressure loss during heating operation is rarely a simple fix

But understanding the causes helps you respond appropriately. While basic checks and repressurisation are within most homeowners' capabilities, the underlying causes – particularly expansion vessel and PRV issues – require professional expertise.

Remember: consistent pressure loss isn't just inconvenient – it can indicate safety-critical component failures that require immediate professional attention.